Investment securities - Schedule of Gross Unrealized Losses (Details) - USD ($) $ in Thousands	Mar. 31, 2020	Dec. 31, 2019
Debt Securities, Available-for-sale [Abstract]
Debt securities, available-for-sale, unrealized loss, less than 12 months, fair value	$ 41,948	$ 63,074
Debt securities, available-for-sale, continuous unrealized loss position, less than 12 months, accumulated loss	(395)	(394)
Debt securities, available-for-sale, continuous unrealized loss position, 12 months or longer, fair value	0	175,730
Debt securities, available-for-sale, continuous unrealized loss position, 12 months or longer, accumulated loss	0	(1,497)
Debt securities, available-for-sale, continuous unrealized loss position, fair value	41,948	238,804
Debt securities, available-for-sale, unrealized loss position, accumulated loss	(395)	(1,891)
Mortgage-backed securities - residential
Debt Securities, Available-for-sale [Abstract]
Debt securities, available-for-sale, unrealized loss, less than 12 months, fair value	1,481	47,641
Debt securities, available-for-sale, continuous unrealized loss position, less than 12 months, accumulated loss	(21)	(164)
Debt securities, available-for-sale, continuous unrealized loss position, 12 months or longer, fair value	0	175,730
Debt securities, available-for-sale, continuous unrealized loss position, 12 months or longer, accumulated loss	0	(1,497)
Debt securities, available-for-sale, continuous unrealized loss position, fair value	1,481	223,371
Debt securities, available-for-sale, unrealized loss position, accumulated loss	(21)	(1,661)
Municipals, tax exempt
Debt Securities, Available-for-sale [Abstract]
Debt securities, available-for-sale, unrealized loss, less than 12 months, fair value	39,482	15,433
Debt securities, available-for-sale, continuous unrealized loss position, less than 12 months, accumulated loss	(359)	(230)
Debt securities, available-for-sale, continuous unrealized loss position, 12 months or longer, fair value	0	0
Debt securities, available-for-sale, continuous unrealized loss position, 12 months or longer, accumulated loss	0	0
Debt securities, available-for-sale, continuous unrealized loss position, fair value	39,482	15,433
Debt securities, available-for-sale, unrealized loss position, accumulated loss	(359)	(230)
Treasury securities
Debt Securities, Available-for-sale [Abstract]
Debt securities, available-for-sale, unrealized loss, less than 12 months, fair value		0
Debt securities, available-for-sale, continuous unrealized loss position, less than 12 months, accumulated loss		0
Debt securities, available-for-sale, continuous unrealized loss position, 12 months or longer, fair value		0
Debt securities, available-for-sale, continuous unrealized loss position, 12 months or longer, accumulated loss		0
Debt securities, available-for-sale, continuous unrealized loss position, fair value		0
Debt securities, available-for-sale, unrealized loss position, accumulated loss		$ 0
Corporate securities
Debt Securities, Available-for-sale [Abstract]
Debt securities, available-for-sale, unrealized loss, less than 12 months, fair value	985
Debt securities, available-for-sale, continuous unrealized loss position, less than 12 months, accumulated loss	(15)
Debt securities, available-for-sale, continuous unrealized loss position, 12 months or longer, fair value	0
Debt securities, available-for-sale, continuous unrealized loss position, 12 months or longer, accumulated loss	0
Debt securities, available-for-sale, continuous unrealized loss position, fair value	985
Debt securities, available-for-sale, unrealized loss position, accumulated loss	$ (15)

No definition available.

Name:	us-gaap_AvailableForSaleSecuritiesAbstract
Namespace Prefix:	us-gaap_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

- Definition

Amount of investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale), in continuous unrealized loss position for more than 12 months, without allowance for credit loss. Includes beneficial interest in securitized financial asset.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 326
-SubTopic 30
-Section 50
-Paragraph 5
-URI http://asc.fasb.org/extlink&oid=118955579&loc=SL82922890-210455

Reference 2: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 50
-Paragraph 7
-URI http://asc.fasb.org/extlink&oid=118939692&loc=d3e27337-111563

Reference 3: http://www.xbrl.org/2003/role/exampleRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 326
-SubTopic 30
-Section 55
-Paragraph 8
-URI http://asc.fasb.org/extlink&oid=118942371&loc=SL82922954-210456

+ Details

Name:	us-gaap_DebtSecuritiesAvailableForSaleContinuousUnrealizedLossPosition12MonthsOrLonger
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Amount of accumulated unrealized loss on investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale), in continuous unrealized loss position for 12 months or longer, without allowance for credit loss. Includes beneficial interest in securitized financial asset.

+ References

+ Details

Name:	us-gaap_DebtSecuritiesAvailableForSaleContinuousUnrealizedLossPosition12MonthsOrLongerAccumulatedLoss
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Amount of investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale), in continuous unrealized loss position for less than 12 months, without allowance for credit loss. Includes beneficial interest in securitized financial asset.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 50
-Paragraph 7
-URI http://asc.fasb.org/extlink&oid=118939692&loc=d3e27337-111563

Reference 2: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 326
-SubTopic 30
-Section 50
-Paragraph 5
-URI http://asc.fasb.org/extlink&oid=118955579&loc=SL82922890-210455

Reference 3: http://www.xbrl.org/2003/role/exampleRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 326
-SubTopic 30
-Section 55
-Paragraph 8
-URI http://asc.fasb.org/extlink&oid=118942371&loc=SL82922954-210456

+ Details

Name:	us-gaap_DebtSecuritiesAvailableForSaleContinuousUnrealizedLossPositionLessThan12Months
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Amount of accumulated unrealized loss on investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale), in continuous unrealized loss position for less than 12 months, without allowance for credit loss. Includes beneficial interest in securitized financial asset.

+ References

+ Details

Name:	us-gaap_DebtSecuritiesAvailableForSaleContinuousUnrealizedLossPositionLessThan12MonthsAccumulatedLoss
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Amount of investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale), in unrealized loss position without allowance for credit loss.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 50
-Paragraph 6
-Subparagraph (a)(1)
-URI http://asc.fasb.org/extlink&oid=118939692&loc=d3e27290-111563

Reference 2: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 326
-SubTopic 30
-Section 50
-Paragraph 4
-Subparagraph (a)(1)
-URI http://asc.fasb.org/extlink&oid=118955579&loc=SL82922888-210455

Reference 3: http://www.xbrl.org/2003/role/exampleRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 326
-SubTopic 30
-Section 55
-Paragraph 8
-URI http://asc.fasb.org/extlink&oid=118942371&loc=SL82922954-210456

+ Details

Name:	us-gaap_DebtSecuritiesAvailableForSaleUnrealizedLossPosition
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Amount of accumulated unrealized loss on investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale), in unrealized loss position, without allowance for credit loss. Includes beneficial interest in securitized financial asset.

+ References

+ Details

Name:	us-gaap_DebtSecuritiesAvailableForSaleUnrealizedLossPositionAccumulatedLoss
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Details

Name:	us-gaap_FinancialInstrumentAxis=us-gaap_ResidentialMortgageBackedSecuritiesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancialInstrumentAxis=fbk_MunicipalsTaxExemptMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancialInstrumentAxis=us-gaap_USTreasurySecuritiesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancialInstrumentAxis=us-gaap_CorporateDebtSecuritiesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type: